Since there will never be instances of call back interfaces, there's no need for a .prototype property on their interface object (which exists only when there are constants on the call back interface).

This is 100% inline with IE9's behavior, so I'm happy to see this minor change. (At the time, we created an extended attribute to control this, but with 'callback' it's not necessary :)
// Introduced in DOM Level 2:
[msInterfaceObjectOnly]
interface NodeFilter
{ /* ... */ };

That line doesn't say that callback interfaces _can't_ have such a property (jus that non-callback ones have to), and other places in the spec are written as if that property is never null....
I think it would be good to explicitly clarify this.